Adiponitrile (ADN) is a commercially important and versatile intermediate in the industrial production of nylon polyamides useful in forming films, fibers, and molded articles. ADN may be produced by hydrocyanation of 1,3-butadiene (BD) in the presence of transition metal complexes comprising various phosphorus-containing ligands. For example, catalysts comprising zero-valent nickel and monodentate phosphorus-containing ligands are well documented in the prior art; see, for example, U.S. Pat. Nos. 3,496,215; 3,631,191; 3,655,723 and 3,766,237; and Tolman, C. A., McKinney, R. J., Seidel, W. C., Druliner, J. D., and Stevens, W. R., Advances in Catalysis, 1985, Vol. 33, pages 1-46. Improvements in the hydrocyanation of ethylenically unsaturated compounds with catalysts comprising zero-valent nickel and certain multidentate phosphite ligands are also disclosed; e.g., see: U.S. Pat. Nos. 5,512,696; 5,821,378; 5,959,135; 5,981,772; 6,020,516; 6,127,567; and 6,812,352.
3-Pentenenitrile (3PN) may be formed through a series of reactions as illustrated below.

According to abbreviations used herein, BD is 1,3-butadiene, HC≡N is hydrogen cyanide, and 2M3BN is 2-methyl-3-butenenitrile. A method to increase the chemical yield of 3PN from BD hydrocyanation includes the catalytic isomerization of 2M3BN to 3PN (Equation 2 above) in the presence of NiL4 complexes as disclosed in U.S. Pat. No. 3,536,748. Co-products of BD hydrocyanation and 2M3BN isomerization may include 4-pentenenitrile (4PN), 2-pentenenitrile (2PN), 2-methyl-2-butenenitrile (2M2BN), and 2-methylglutaronitrile (MGN).
In the presence of transition metal complexes comprising various phosphorus-containing ligands, dinitriles such as ADN, MGN, and ethylsuccinonitrile (ESN) may be formed by the hydrocyanation of 3PN and 2M3BN, as illustrated in Equations 3 and 4 below. Equation 4 also shows that 2M2BN can be formed when 2M3BN undesirably isomerizes in the presence of a Lewis acid promoter that may be carried over from a pentenenitrile hydrocyanation reaction zone.

The hydrocyanation of activated olefins such as conjugated olefins (e.g., 1,3-butadiene) can proceed at useful rates without the use of a Lewis acid promoter. However, the hydrocyanation of un-activated olefins, such as 3PN, require at least one Lewis acid promoter to obtain industrially useful rates and yields for the production of linear nitriles, such as ADN. For example, U.S. Pat. Nos. 3,496,217, 4,874,884, and 5,688,986 disclose the use of Lewis acid promoters for the hydrocyanation of non-conjugated ethylenically unsaturated compounds with nickel catalysts comprising phosphorous-containing ligands.
An integrated process for the production of ADN from BD and HC≡N can comprise BD hydrocyanation, 2M3BN isomerization to produce 3PN, and the hydrocyanation of pentenenitriles, including 3PN, to produce ADN and other dinitriles. Integrated processes are disclosed, for example, in United States Patent Application 2009/0099386 A1.
Disclosed in United States Patent Publication No. 2007/0260086, is a process for the preparation of dinitriles with an aim to provide for the recovery of a catalyst formed by a mixture of mono- and bidentate ligands and to be able to reuse the catalyst thus recovered in the hydrocyanation and/or isomerization stages.
United States Patent Publication No. 2008/0221351 discloses an integrated process for preparing ADN. A first process step includes hydrocyanating BD to produce 3PN over at least one zero-valent nickel catalyst. A second process step of the integrated process involves hydrocyanating 3PN to produce ADN over at least one zero-valent nickel catalyst and at least one Lewis acid. In this integrated process, at least one of the zero-valent nickel catalysts used in one of the process steps is transferred into the other process step.
United States Patent Application Publication 2007/0155978 discloses a method for recovering a catalyst from an extract by distillation. In the removal of the extractant to recover the catalyst, in a preferred embodiment, 3-pentenenitrile is added to the distillation as an intermediate boiler. One advantage of this solvent change is that effective depletion of the extractant from the high-boiling catalyst stream is possible at evaporator temperatures which are low enough not to thermally damage the particular nickel catalyst used and especially the chelate ligand. The pressure is still high enough to be able to condense the extractant having a comparatively low boiling point in comparison to the catalyst constituents at the top of the evaporator stage or distillation column even at customary cooling water temperatures of from 25 to 50° C.
United States Patent Application Publication 2007/0155979 describes a process for the hydrocyanation of unsaturated compounds to unsaturated mononitrile compounds or to dinitrile compounds. The reaction medium obtained after the hydrocyanation reaction is advantageously subjected to separation by distillation of the unreacted reactant, namely butadiene or the unsaturated nitrile, in order to be recycled. These separation stages are carried out while observing a distillation bottom temperature according to the conditions of ligand/nickel ratio and nickel concentration in order to avoid or limit decomplexing of the nickel and its precipitation.
United States Patent Application Publication 2009/0187039 describes a multistage process for distilling the effluent from a hydrocyanation reactor for reacting 1,3-butadiene with hydrogen cyanide. In one process step, an evaporator stage associated with a distillation apparatus is designed in such a way that the material to be evaporated is subject to very little thermal damage, as achieved, for example, by falling-film evaporators, multiphase helical tube evaporators, thin-film evaporators or short-path evaporators by short contact times of the material on the evaporator surface and very low temperatures of the evaporator surfaces. In a further preferred embodiment of the process, the distillation is carried out at average residence times of the liquid phase in the bottom region of the distillation apparatus of together less than 10 hours, more preferably less than 5 hours, in particular less than 1 hour. In a particularly preferred embodiment of the process, the distillation is carried out at average residence times of the liquid phase in the bottom region of the distillation apparatus in process steps of together less than 10 hours, more preferably less than 5 hours, in particular less than 1 hour. The absolute pressure in one process step is preferably from 0.001 to 10 bar, more preferably from 0.010 to 1 bar, in particular from 0.020 to 0.5 bar. The distillation is carried out in such a way that the temperature in the bottom of the distillation apparatus is preferably from 30 to 140° C., more preferably from 40 to 130° C., in particular from 50 to 120° C. The distillation is carried out in such a way that the condensation temperature at the top of the distillation apparatus is preferably from −20 to 140° C., more preferably from −10 to 80° C., in particular from −5 to 60° C. In a particularly preferred embodiment of the process, the aforementioned temperature ranges are maintained both at the top and in the bottom of the distillation apparatus.
It is reported in the prior art that, concomitant with the hydrocyanation of 3PN and 4PN to produce ADN, some isomerization of 3PN to cis- and trans-2PN can occur. However, in the process of hydrocyanating 3PN and 4PN using nickel catalysts derived from monodentate phosphite ligands, such as Ni[P(OC6H5)3]4, U.S. Pat. No. 3,564,040 states that the presence of 2PN, even in low concentrations, is detrimental to catalyst efficiency and the production of 2PN is undesirable since this presence and production of 2PN constitute a yield loss as well as a poison for the catalyst.
In order to address this issue, U.S. Pat. No. 3,564,040 describes a method to maintain the steady-state concentration of 2PN below 5 mole percent as based on the nitriles present in the reaction mixture. Because trans-2PN is difficult to separate from a mixture of 3PN and 4PN by distillation due to their close relative volatilities, the disclosed method involves the catalytic isomerization of trans-2PN to cis-2PN followed by fractional distillation of the mixture of pentenenitrile isomers to remove the more volatile cis-2PN Isomer. The catalyst systems used to isomerize trans-2PN to cis-2PN are those that also serve to hydrocyanate pentenenitriles to ADN, in particular, nickel catalysts derived from monodentate phosphite ligands as described in U.S. Pat. Nos. 3,496,217 and 3,496,218.
Alternative catalyst systems for the isomerization of trans-2PN to cis-2PN are disclosed in U.S. Pat. Nos. 3,852,325 and 3,852,327. The primary advantage of the catalyst systems described therein is in avoiding appreciable carbon-carbon double bond migration in the pentenenitrile isomers, which allows for the isomerization of trans-2PN to cis-2PN without substantial further isomerization of the 3PN to 2PN. The catalysts described in U.S. Pat. No. 3,852,325 are compounds of the general formula R3C—X, such as triphenylmethyl bromide, wherein R is an aryl radical having up to 18 carbon atoms and —X is of the group consisting of —H, —Cl, —Br, —I, —SH, —B(C6H5)4, —PF6, —AsF6, —SbF6 and —BF4, while the catalyst systems described in U.S. Pat. No. 3,852,327 are Lewis acid/Lewis base compositions, such as combinations of zinc chloride with triphenylphosphine.
A different method of removing the 2PN from mixtures of pentenenitrile isomers containing 3PN and 4PN is disclosed in U.S. Pat. No. 3,865,865. The 2PN and/or 2-methyl-2-butenenitriles (2M2BN) can be selectively separated from a mixture of pentenenitrile isomers containing 3PN and 4PN by contacting the mixture of nitriles with an aqueous solution of a treating agent comprising sulfite and bisulfite ions and ammonium or alkali metal cations to produce an aqueous phase containing the bisulfite adduct of the 2PN and/or 2M2BN and an organic phase containing the 3PN and 4PN, substantially free of 2PN and 2M2BN. The recovered organic phase is said to provide a feed material of pentenenitriles for further hydrocyanation to produce ADN with greatly reduced amounts of the undesired by-product 2PN, which is said to be detrimental to catalyst efficiency.
U.S. Pat. No. 6,127,567 discloses nickel catalyst compositions derived from bidentate phosphite ligands and processes for the hydrocyanation of monoethylenically unsaturated compounds which are said to be more rapid, selective, efficient, and stable than prior processes using nickel catalysts derived from monodentate phosphites. U.S. Pat. No. 5,688,986 discloses that at least one member of this class of catalysts is capable of hydrocyanating olefins conjugated to nitriles, for example 2PN.
U.S. Pat. No. 8,088,943 describes a process for the hydrocyanation of 3-pentenenitriles to produce ADN, using certain catalyst compositions described in U.S. Pat. No. 6,127,567 as well as other catalyst compositions.
U.S. Pat. No. 8,088,943 also describes a process for refining the reaction product mixture to obtain, for example, a stream comprising adiponitrile, a stream comprising a catalyst composition, and a stream comprising ethylenically unsaturated nitriles. The hydrocyanation process involves introducing 2-pentenenitrile along with 3-pentenenitrile as a feed to a hydrocyanation reactor to produce adiponitrile. The product from the hydrocyanation reactor is passed to an extraction step, for example, as described in U.S. Pat. No. 3,773,809. In the description of U.S. Pat. No. 8,088,943, the reaction product from the hydrocyanation reactor is passed directly to the extraction step without an intermediate distillation step to remove unreacted 3-pentenenitrile from the reaction product mixture.
U.S. Pat. No. 3,773,809 discloses a process for separating an organic phosphorus compound or a zerovalent nickel complex of the organic phosphorus compound from the reaction product of a hydrocyanation reaction of 3-pentenenitrile with hydrogen cyanide. The reaction product is contacted with a paraffin or cycloparaffin hydrocarbon solvent at a temperature of about 0° C. to about 100° C. to produce a multiphase mixture, wherein the organic phosphorus compounds and their metal complexes are contained predominantly in the hydrocarbon phase (i.e. the light phase) and the organic mono- and dinitrile and degradation products are contained predominately in a separate phase (i.e. a heavy or raffinate phase). In Example 1, the hydrocarbon solvent was cyclohexane (i.e. cyane), the hydrocarbon phase included 5.16 wt % of pentenenitriles, and the raffinate phase included 24.0 wt % of pentenenitriles. In Example 6, the hydrocarbon solvent was cyclohexane (i.e. cyane), and essentially all of the 2-pentenenitriles (i.e. trans-2-pentenenitrile and cis-2-pentenenitrile) were apparently found in the raffinate phase with no 2-pentenenitriles being reported in the hydrocarbon phase.
U.S. Pat. No. 7,816,551 describes a process for the hydrocyanation of 3-pentenenitrile to produce ADN, followed by distillation step to remove a portion of the unreacted 3-pentenenitrile from the hydrocyanation reaction product mixture, followed, in turn, by an extraction step to remove catalyst from the distilled reaction product mixture. The unreacted 3-pentenenitrile is removed from the distillation step in an overhead stream, and the catalyst is removed from the distillation step as a bottoms stream.
In addition to unreacted 3-pentenenitrile, the overhead stream comprises 2-pentenenitrile and (E)-2-methyl-2-butenenitrile. According to the description of U.S. Pat. No. 7,816,551, this overhead stream may be distilled to remove cis-2-pentenenitrile and (E)-2-methyl-2-butenenitrile prior to recycling the unreacted 3-pentenenitrile to the hydrocyanation reactor.
U.S. Pat. No. 7,816,551 further describes recovering unreacted 3-pentenenitrile from a raffinate stream obtained from the extraction step to remove catalyst from the distilled reaction product mixture. The raffinate stream is first distilled to remove residual extraction solvent from the stream. This distilled raffinate stream is then further distilled to remove pentenenitriles, comprising 3-pentenenitrile, 2-pentenenitrile and 2-methyl-2-butenenitrile as an overhead stream. This overhead stream may then be further distilled to remove cis-2-pentenenitrile and 2-methyl-2-butenenitrile in an overhead stream and to recover 3-pentenenitrile in a bottoms stream. The recovered 3-pentenenitrile may then be recycled to the hydrocyanation reactor. The above mentioned two distillation steps to remove cis-2-pentenenitrile and 2-methyl-2-butenenitrile from 3-pentenenitrile may take place in the same distillation apparatus.